"hey grumpy, my cars new 396 big block engine with a 750 Holley got a very noticeable just off idle stumble, if I try to accelerate smoothly barely giving the engine some gas it runs rough, if I stomp on the throttle theres a slight bog but then its rapidly pulling and off we go, but part throttle in traffic always tends to run rough, any ideas?"
I tune a great many cars and given your description of the symptom,s here is what I think is occurring, the stumble is likely caused by a momentary lean condition during the transition, if you slowly and gradually increase throttle , if you hit the throttle a bit harder the accelerator pump covers and masked the problem.you might have a vacuum leak, or the carbs just not tuned correctly with the required power valve , jets and accelerator pump cam and/or linkage adjustment, you might need to play with the jet sizes or ignition curve also, and you darn sure need to make sure the fuel pressure consistent at about 5 psi, the floats are set correctly and yes if I had access to the car I could quickly walk you thru a few tests.
but since Im not there Id like you to post CLEAR pictures of the spark plugs and tell me the jet sizes, the power valves and the color of the accelerator pump cam, and what size squirters your using, the type of intake and if your using any spacers .
you need to start by setting the floats and verifying the fuel pressures correct then move to and verify the accelerator pump linkage is working and check for vacuum leaks, each link below has answers, read thru them
reading thru these links will help you isolate the problems cause
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=109
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=1639
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=5229
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=5229
http://www.secondchancegarage.com/public/186.cfm
viewtopic.php?f=44&t=882&p=1390&hilit=propane+leaks#p1390
viewtopic.php?f=70&t=202
viewtopic.php?f=70&t=4683
I tune a great many cars and given your description of the symptom,s here is what I think is occurring, the stumble is likely caused by a momentary lean condition during the transition, if you slowly and gradually increase throttle , if you hit the throttle a bit harder the accelerator pump covers and masked the problem.you might have a vacuum leak, or the carbs just not tuned correctly with the required power valve , jets and accelerator pump cam and/or linkage adjustment, you might need to play with the jet sizes or ignition curve also, and you darn sure need to make sure the fuel pressure consistent at about 5 psi, the floats are set correctly and yes if I had access to the car I could quickly walk you thru a few tests.
but since Im not there Id like you to post CLEAR pictures of the spark plugs and tell me the jet sizes, the power valves and the color of the accelerator pump cam, and what size squirters your using, the type of intake and if your using any spacers .
you need to start by setting the floats and verifying the fuel pressures correct then move to and verify the accelerator pump linkage is working and check for vacuum leaks, each link below has answers, read thru them
reading thru these links will help you isolate the problems cause
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=109
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=1639
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=5229
http://forum.grumpysperformance.com/viewtopic.php?f=55&t=5229
http://www.secondchancegarage.com/public/186.cfm
viewtopic.php?f=44&t=882&p=1390&hilit=propane+leaks#p1390
viewtopic.php?f=70&t=202
viewtopic.php?f=70&t=4683